Just in case it is some kind of toxin, start by adding some activated charcoal or purigen to give you some time while you figure out the cause.
How many episodes have you had where the fish died like this? How far apart were they? Is there a chance someone has been spraying air freshener or that someone other than you might have accidentally done something to the tank or spilled something on your equipment? Since the prefilter is the only new thing you've added, try rinsing it very thoroughly under running tap water just in case.
Also, is this happening shortly before or after your water changes? If it's happening within a day of a water change, it's possible your water company has added something or there's some kind of contaminate in the water supply. You can look up your city's water report online.
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